Schmitt trigger circuit

ABSTRACT

A Schmitt trigger circuit has a complimentary set of normally non-conductive transistors. The first transistor is of the NPN type and the second transistor is of th PNP type. A single source of voltage energizes the circuit. The collector of the first transistor is connected to the base of the second transistor through a current limiting resistor. A feedback voltage divider connected to the collector of the second transistor has a center tap which is connected through a current limiting resistor to the base of the first transistor. The transistors are so arranged so that both transistors are normally non-conductive in the standby mode - this preventing a drain of current from the source of voltage. When a varying D.C. voltage applied to the base of the first transistor rises above a predetermined value, both transistors become conductive and the voltage between the collector of the second transistor and the ground reference potential of the circuit, to which an external load may be connected, switches from zero voltage to substantially the full power supply voltage. When the trigger circuit is utilized to sense A.C. voltages, a rectifier is provided at the input to the above described circuit, namely at the base of the first transistor, which first rectifies the varying or fluctuating A.C. voltages to varying D.C. voltages. A diode current return path resistor is connected between the diode and the center tap of the feedback voltage divider to further decrease switching times.

United States Patent [191 Meri [ June 11, 1974 SCHMITT TRIGGER CIRCUIT [75] Inventor: Kaliu Merl, Maspeth, NY.

[73] Assignee: Electrospace Corporation, North Bergen, NJ.

22 Filed: Mar. 23, 1973 21 Appl. No.: 344,124

[52] US. Cl. 307/288, 307/290 [51] Int. Cl. H03k 3/286, H03k 3/295 [58] Field of Search 307/238, 255, 272, 288, 307/289, 290, 291, 292, 313

[56] References Cited UNITED STATES PATENTS 3,121,802 2/1964 Palmer 307/288 3,215,852 11/1965 Brode et a1.. 307/288 X 3,292,005 12/1966 Lee 307/288 X 3,319,086 5/1967 Yee 307/288 3,364,391 1/1968 Jensen 307/288 X 3,373,315 3/1968 Colman 307/288 X 3,374,366 3/1968 Kleinberg 307/288 3,376,434 4/1968 Weinstock 307/288 X 3,418,528 12/1968 Watanabe 307/289 X 3,466,506 9/1969 Badovinac et a1. 307/288 X 3,487,316 12/1969 Winder 307/290 X FOREIGN PATENTS OR APPLICATIONS 4,312,291 6/1968 Japan 307/290 Primary Examiner-Rudolph V. Rolinec Assistant ExaminerL. N. Anagnos Attorney, Agent, or F [rm-Friedman & Goodman [5 7 ABSTRACT A Schmitt trigger circuit has a complimentary set of normally non-conductive transistors. The first transistor is of the NPN type and the second transistor is of th PNP type. A single source of voltage energizes the circuit. The collector of the first transistor is connected to the base of the second transistor through a current limiting resistor. A feedback voltage divider connected to the collector of the second transistor has a center tap which is connected through a current limiting resistor to the base of the first transistor. The transistors are so arranged so that both transistors are normally non-conductive in the standby mode this preventing a drain of current from the source of voltage. When a varying DC. voltage applied to the base of the first transistor rises above a predetermined value, both transistors become conductive and the voltage between the collector of the second transistor and the ground reference potential of the circuit, to which an external load may be connected, switches from zero voltage to substantially the full power supply voltage. When the trigger circuit is utilized to sense A.C. voltages, a rectifier is provided at the input to the above described circuit, namely at the base of the first transistor, which first rectifies the varying or fluctuating A.C. voltages to varying D.C. voltages. A diode current return path resistor is connected between the diode and the center tap of the feedback voltage divider to further decrease switching times.

10 Claims, 2 Drawing Figures SCHMITT TRIGGER CIRCUIT BACKGROUND OF THE INVENTION The present invention generally relates to trigger circuits, and more particularly it relates to a Schmitt trigger circuit with full power supply output swing and zero standby current.

Triggering circuits of various types are already known. Also known are a class of trigger circuits known as Schmitt trigger circuits whose switching action is a function of predetermined levels of a fluctuating or varying voltage at its input. Schmitt triggers have a standby mode wherein the active components in the circuits, such a transistors, assume normal conductive states when a DC. voltage level at the input to the trigger circuit falls below a predetermined value. So long as the voltage of the input remains below the first predetermined level or value, the active components remain in their normal states. However, as soon as the input level surpasses a second predetermined level, normally higher in value than the first predetermined level, the conductive states of the active components change this manifesting itself in switching action at an output wherein an external load is connected. The levels of input voltage at which the active elements change states, or the levels at which the output voltage switches across an external load, are termed the trip points of the trigger circuit. The difference between the input voltage levels for switching the trigger circuit to its active mode and that for switching the trigger circuit back to its inactive or standby mode is termed the hysteresis of the circuit.

The trigger circuits of the prior art, however, have numerous problems associated therewith. Firstly, many of the prior art trigger circuits include one or more active elements in the circuit which are in conductive states in the standby mode. Accordingly, the trigger circuit draws current from a source of power, sometimes for long periods of time in which the circuit remains in the standby mode. This represents a waste of power and inefficient operation, and is a problem particularly acute when the trigger circuit is utilized in connection with a battery-operated device.

In order to minimize current drain, some prior art trigger circuits have utilized more than one power supply, e.g., positive and negative supplies. Other trigger circuits utilize complex combinations of active and passive elements. Such complex circuits are expensive to manufacture and are more subjected to frequent breakdown.

Another problem associated with many prior art trigger circuits is the limited voltage output swing which it provides across an external load. It is normally desired to have a maximum voltage output swing in which the switching takes place between the reference or ground potential and a value substantially equal to the voltage of the power supply. As above noted, many of the prior art supplies do not provide such a large voltage swing approximating the value of the power supply voltage. Such prior art trigger circuits, in an effort to increase the voltage output swing, have utilized more than one power supply. This has increased the cost of manufacturing as well as of utilizing such circuits.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a trigger circuit which does not have the above described disadvantages associated with prior art trigger circuits.

It is another object of the present invention to provide a trigger circuit which is simple in construction and economical to manufacture.

It is still another object of the present invention to provide a triggered circuit of the type under discussion which draws zero current from the power supply in the standby mode.

It is yet another object of the present invention to provide a trigger circuit as above described which need only utilize one power supply.

It is a further object of the present invention to provide a trigger circuit which is utilized with a single power supply and which provides output switching between a zero level and a level substantially equal to the voltage associated with the power supply.

It is an additional object of the present invention to provide a trigger circuit which utilizes a very low number of active as well as passive components, thereby making the circuit reliable.

It is still an additional object of the present invention to provide a trigger circuit whose output voltage swing is substantially independent of the load resistancd.

It is yet an additional object of the present invention to provide a trigger circuit of the type under discussion in which the trip points as well as the hysteresis of the circuit are substantially independent of the gain of the transistors in the circuit as well as the value of the external load resistance.

It is a further additional object of the present invention to provide a triggering circuit which includes a rectifier for rectifying A.C. signals at the input of the circuit, the rectifier having a resistor associated therewith which is connected to a positive feedback voltage divider to thereby decrease the switching times of the trigger circuit.

In order to achieve the above objects as well as others which will become apparent hereafter, the Schmitt trigger circuit in accordance with the present invention comprises D.C. input terminal means adapted to be connected to a source of DC. varying voltage. A set of normally non-conducting transistors are provided, the set comprising a first transistor and a second transistor. The base of the first transistor is connected to said D.C. input terminal means. Coupling means are provided which is connected between said first and second transistors for rendering the second transistor conductive whenever said first transistor is conductive. Connecting means are provided for connecting a source of electrical energy to said transistors. Output terminal means are connected to said second transistor and are adapted to be connected to an external load. Feedback means are provided and are connected between said base of said first transistor and said output terminal means for providing positive feedback therebetween. Said transistors are arranged to supply substantially the entire voltage associated with said source of electrical energy to said output terminal means when the varying D.C. voltage assumes a first predetermined value and said first and second transistors are conductive. On the other hand, said transistors prevents voltage from being applied to said output terminal means and prevent current from being drawn from the source of electrical energy by the trigger circuit and the external load when the varying DC. voltage assumes a second predetermined value and said transistors are non-conductive.

According to a presently preferred embodiment, said first and second transistors form a complimentary pair of transistors. For example, said first transistor can be a NPN transistor, while said second transistor is a PNP transistor. The collector of the first transistor is connected to the base of the second transistor through a current limiting resistor.

The feedback means comprises a voltage divider means connected between the collector of said second transistor and the ground, and resistance means connected between the voltage divider means and the base of the first transistor.

According to a further embodiment, the above described trigger circuit is provided with A.C. terminal means to be connected to a source of varying A.C voltage. Rectifying means is interposed between said A.C. and DC. terminal means for rectifying an A.C. voltage at said A.C. input terminal means into a DC voltage at said D.C. terminal means.

Advantageously, in connection with the last described embodiment, said feedback means comprises a voltage divider having a center tap, said voltage divider being connected between the collector of said second transistor and the circuit ground. Said rectifying means comprises resistance means connected to the reference potential through said center tap of the voltage divider.

BRIEF DESCRIPTION OF THE DRAWINGS With the above and additional objects and advantages in view, as will hereinafter be appear, this invention comprises the devices, combinations and arrangements of parts hereinafter described and illustrated in the accompanying drawings of a preferred embodiment in which:

FIG. 1 is a schematic representation of a trigger circuit in accordance with the present invention which is suitable to be connected to a source of DC. varying voltage; and

FIG. 2 is a schematic representation of a further embodiment of the present invention suitable for being connected to a source of A.C. varying voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, in which the same or similar parts have been designated by the same reference numerals, and first referring to FIG. 1, a trigger circuit is shown schematically and generally designated by the reference numeral 10. The trigger circuit has input terminals l2, 14. Since trigger circuits of this type generally operate on varying or fluctuating D.C. voltages, the input terminal 12, 14 are termed, for purposes of the specification as well as the claims, as the DC. input terminals. The input terminals l2, 14 are adapted, in any conventional manner, to be connected to a source of varying or fluctuating DC. voltage.

The circuit 10 has a transistor 20, shown to be of the NPN type, which has a base 22 connected to the input terminal 12 through a current limiting resistor 24. The collector 26 of the transistor is connected to a power supply terminal 27 through a load or collector resistor 28. The power supply terminal 27 is adapted to be connected, in any conventional manner, to the positive pole of a power supply having a voltage +V. The negative pole of the power supply is connectable to the circuit or reference potential, i.e., the circuit ground,

which is designated in FIG. 1 by the reference numeral 15. The emitter 30 of the transistor 20 is connected directly to the circuit ground 15.

The circuit 10 comprises a further transistor 40, shown to be of the PNP type. The transistor 40 has its base 42 connected to the collector 26 of the transistor 20 through a current limiting resistor 44. The emitter of the transistor 40 is connected directly to the power supply terminal 27 while the collector 48 of the transistor 40 is connected to the circuit ground 15 through a voltage divider 49.

It should be noted that to minimize the number of active elements in the trigger circuit 10, a complimentary pair or set of transistors 20, 40 are advantageously utilized. It should be clear that the specific types of the complimentary transistors is not critical for the purpose of the present invention and the transistor types can be changed if the circuit is modified, both as to component arrangements and power supply polarities in a manner well known to those skilled in the art.

The voltage divider 49 comprises two series connected resistors 50, 52 which together form a center tap 54. The voltage divider 49 serves as a load for transistor 40 while simultaneously serving as a positive feedback mechanism in cooperation with a current limiting resistor 56 which is connected between the center tap 54 and the base 22 of the transistor 20.

An output terminal 60 is provided which is connected to the collector 48 of the transistor 40 while an output terminal 62 is connected to the circuit ground 15. An external load 64, shown in dashed outline, is connectable to the output terminals 60, 62 in any conventional manner.

Referring now to FIG. 2, a modified embodiment of the trigger circuit 10 is shown for use with A.C. signals having varying or fluctuating amplitudes. As noted above, Schmitt triggers are basically D.C. circuits. Consequently, the modified trigger circuit 10' in FIG. 2 is identical in all respects with the trigger circuit 10 except for the addition or rectifying means at the input end of the circuit. The DC. input terminals 12, 14 in the basic circuit 10 are similarly designated in FIG. 2. The circuitry that follows the DC. input terminals 12, 14 is identical in each embodiment. The rectifying means, for converting the varying or fluctuating A.C. voltages to corresponding D.C. voltages, includes A.C. input terminal means 70, 72. A source of varying A.C. voltage is connectable to the A.C. input terminals 70, 72 in any conventional manner. The input terminal 72 is, similarly to the input terminal 14, connected to the circuit ground 15.

Connected in series between the A.C. input terminal and the DC input terminal 12 is a matching resistor 74, a decoupling capacitor 76 and a rectifier diode 78. The anode of the diode 78 is connected to the DC. input terminal 12 while its cathode is connected to one terminal of the capacitor 76.

An important feature of the present invention is the provision of a diode current return path through a resistor 82 which is not, however, connected to the circuit ground 15 but, instead, is connected to the feedback center tap 54. In this manner, the total feedback is enhanced and switching times are correspondingly decreased.

The matching resistor 74 is optional and is merely included for matching purposes as well as for preventing overload of the circuit which is connected to the A.C.

input terminals 70, 72. The decoupling capacitor 76 has for its primary function the prevention of D.C. levels which may exist in the A.C. voltage source circuit from influencing the D.C. levels at the D.C. input terminal 12. In this manner, the D.C. voltages which appear at the D.C. input terminal 12 are strictly derived from the rectified A.C. voltages and represent fluctuations in the latter. Thus, it should be apparent that the only purpose of the addition of the rectifing means is to convert fluctuating A.C. signals into corresponding D.C. signals which may then be acted upon by the trigger circuit in a manner which is to be described hereafter. The operation of the rectifying means which represents the circuitry added in FIG. 2, is well known to those skilled in the art and does not per se form an important feature of the present invention. However, it is deemed to be an important feature of the present invention to connect the current path or loading resistor 82 to the feedback center tab 54 instead of merely to the circuit ground 15.

The operation of the trigger circuit illustrated in FIG. 1 will now be described. Normally, when no input voltage is applied between the input terminals 12, 14 or a voltage level is applied therebetween which is below the triggering trip point level, the transistors and 40 are both non-conductive. Accordingly, in the standby mode as well as with low-input D.C. voltages, there is no current drawn by the transistors and consequently there is no current drain from the source of electrical energy or power supply connected to the terminal 27.

When the input D.C. voltage applied to the terminals 12, 14 exceeds a predetermined level or triggering trip point, sufficient base current is provided for the transistor 20 and the latter becomes conductive. With the initiation of conduction of the transistor 20, the collector 26 drops from the normally high potential to a low potential substantially equal to the saturation potential of the transistor 20. Since the base 42 of the transistor 40 is connected to the collector 26, the potential at the base 42 drops relative to the fixed positive potential at the emitter 46 and the transistor 40 similarly becomes conductive. The resistance values in the trigger circuit 10 are so selected so that each of the transistors is fully cut off in the non-conductive state while both transistors go into saturation when they become conductive. Accordingly, the emitter-collector path of the transistor 40 drops from its initial supply voltage to a low saturation voltage. Thus, the voltage between the collector 48 and the circuit ground 50 switches from an initial zero voltage to a voltage which is substantially equal to the voltage of the power supply +V. Actually, the voltage at the collector 48 will be equal to the power supply voltage +V less the emitter-collector saturation voltage of the transistor 40.

An external load resistor 64 connected between the output terminals 60, 62 will have impressed thereon a square voltage wave form which essentially changes between zero and thepositive voltage potential of the power supply as the input varying DC. voltage applied between input terminals l2, l4 fluctuates between the trip points.

When the source of D.C. varying voltage drops below the lower trip point, both transistors 20, 40 revert to their non-conductive states and the voltage across the external load resistor 64 again drops to zero. During such time, as pointed out above, there is no current flow from the power supply or source of electrical energy +V through either of the transistors 20, 40 or through the external load. This represents an important current saving feature which is especially important when the trigger circuit is utilized with battery operated devices.

The operation of the trigger circuit of FIG. 1 has been described above in general terms. A specific example of a circuit which has been constructed and tested is defined by the following circuit components and component values of FIG. 2:

CIRCUIT ELEMENT TYPE OR VALUE Transistor 20 2N5l72 Resistor 24 I0 Kilohms Resistor 28 I0 Kilohms Transistor 40 2N4248 Resistor 44 I0 Kilohms Resistor 50 4.7 Kilohms Resistor 52 I00 Ohms Resistor 56 I00 Kilohms Resistor 74 I0 Kilohms Capacitor 76 0.01 Microfarads Resistor 82 47 Kilohms Capacitor 80 0.1 Microfarads The above listed component types and values are merely illustrative of one possible construction of the embodiment illustrated in FIG. 2. Various modifications to suit specific purposes may be made by those skilled in the art. However, all the resulting circuits which encompass the spirit of the present invention as specified in the claims are similarly deemed to be within the scope of the present invention. A triggering circuit for A.C. signal as above specified has been constructed and tested. The data, which is tabulated below, illustrates some of the advantageous characteristics of the trigger circuit in accordance with the present invention.

All the test data has been obtained by application of a +12 volt D.C. power supply voltage to the terminal 27. The input voltages are in rms values for A.C. signals applied between A.C. input terminals and 72. Also, the test data was taken with an A.C. input signal applied to the input terminals 70, 72 which had a test frequency of l kilohertz.

One advantageous characteristic of the present circuit is that the triggering sensitivity is substantially independent of the transistor gain or the D.C. beta of the two transistors 20, 40. The triggering sensitivity as a function of different transistor betas is tabulated as follows:

It will be noted from the above tabulation that the trip points, both for ON and OFF switching conditions, are fairly constant over wide ranges of D.C. beta.

Another advantageous feature of the triggering circuits 10' is the independence of triggering sensitivity on the value of the load resistance 64. Stated otherwise,

the ON and OFF" trip points remain essentially fixed over wide ranges of load resistance. This is illustrated in the following tabulation:

pp y Load Output Drain 100k +l l.93v 3.5mA lOk +1 l.9lv 4.7mA lk +1 l.76 lSmA 680 +1 l.69v 2lmA It will be noted from the above tabulation that changing the load between 680 ohms and 100 kilohms hardly effects the value of the output voltage swing across the load resistor 64. In ach case. the voltage is almost equal to the +12 volt power supply voltage. Of course, the output voltage drops to zero in the standby mode of the trigger circuit 10.

The voltage divider which comprises resistors 50, 52 may be adjusted to vary the voltage at the center tap 54. More particularly, the resistors 50, 52 may be adjusted to adjust the availability of latching. Thus, the adjustment of the voltage divider 49 controls the degree of hysteresis which the trigger circuit will provide either broadening it or narrowing it to provide a hair trigger over a very narrow fluctuation range at the input.

Advantageously, the resistor 50 is maintained at a fixed value while the resistor 52 is changed to provide different degrees of latching. Of course, the resistor 50 may similarly be varied to change the voltage at the center tap 54, with different degrees of advantage.

The percent of latching depends, therefore, with the circuit shown in FIG. 2, on the positive feedback resistor 52 as illustrated in the following tabulation:

Resistor 52 value input rms V holds until signal As can be observed from the above table, the percent of latching can be mostly regulated by controlling the value of the resistor 52. In each case, the ON voltage remains substantially constant while the OFF" voltage varies considerably. Based on these two voltages, the trigger circuit 10' continues to provide a voltage across the load resistor 64 until the in pgt signal is re;

duced to between 22 and 96 percent of the rms voltage which causes the circuit to change in states from its standby mode.

The above described advantages are realized with a simple circuit having only two active devices transistors 20, 40. Consequently, the circuit 10 is highly reliable and economical to manufacture.

Although the above description has been made in connection with FIG. 2, it should be clear that similar operating characteristics hold true for the circuit of FIG. 1, when a DC. fluctuating or varying voltage is applied between the input terminals 12, 14.

Numerous alterations of the structure herein disclosed will suggest themselves to those skilled in the art. However, it is to be understood that the present disclosure relates to a preferred embodiment of the invention which is for purposes of illustration only and not to be constructed as a limitation of the invention.

What is claimed is:

1. A trigger circuit comprising D.C. input terminal means adapted to be connected to a source of DC varying voltage and having a reference potential associated therewith; a set of normally non-conductive transistors comprising a first transistor and a second transistor, the base of said first transistor being connected to said D.C. input terminal means; coupling means connected between said first and second transistors for rendering said said transistor conductive whenever said first transistor is conductive; connecting means for connecting a source of electrical energy to said transistors; output terminal means connected to said second transistor adapted to be connected to an external load; and feedback means connected between said base of said first transistor and said output terminal means for providing positive feedback therebetween, said feedback means comprising voltage divider means connected between the collector of said second transistor and said reference potential, and resistance means connected between said voltage divider means and said base of said first transistor, said transistors being arranged to supply substantially the entire voltage associated with said source of electrical energy to said output terminals when the varying DC. voltage assumes a first predetermined value and said first and second transistors are conductive, while preventing said voltage from being applied to said output terminal means and preventing current from being drawn from the source of electrical energy by the trigger circuit and the external load when the varying DC. voltage assumes a second predetermined value and said transistors are non-conductive.

2. A trigger circuit as defined in claim 1, wherein said first and second transistors form a complimentary pair of transistors.

3. A trigger circuit as defined in claim 2, wherein said first transistor is a PNP transistor while said second transistor is a PNP transistor.

4. A trigger circuit as defined in claim 1, wherein said coupling means comprises a resistor connected between the collector of said first transistor and the base of said second transistor.

5. A trigger circuit as defined in claim 1, further comprising a single source of electrical energy connected to said connecting means.

6. A trigger circuit as defined in claim 1, wherein said voltage divider means comprises two series connected resistors forming a center tap, said resistance means comprising a resistor connected between said center tap and said base of said first transistor.

7. A trigger circuit as defined in claim 1, further comprising A.C. input terminal means adapted to be connected to a source of varying A.C. voltage, and rectifying means interposed between said A.C. and DC. input terminal means for rectifying an A.C. voltage at said A.C. input terminal means into a DC. voltage at said D.C. input to terminal means.

8. A trigger circuit as defined in claim 8, wherein said rectifying means comprises a diode connected between said A.C. and DC. input terminal means; resistance means connected between the cathode of the diode and a reference potential of the trigger circuit; and capacitance means connected between the anode of the diode and the reference potential of the trigger circuit, said resistance means providing a current return path for said diode, while said capacitance means charges to a rectified DC. voltage while filtering the A.C. voltage at the anode of said diode to the reference potential, whereby the voltage which appears at said D.C. input terminal means is substantially a DC voltage devoid of A.C. voltage.

9. A trigger circuit as defined in claim 8, wherein said divider means comprises a voltage divider having a center tap, said voltage divider being connected between the collector of said second transistor and the reference potential, said resistance means comprising a resistor connected to the reference potential through said center tap of said voltage divider.

10. A trigger circuit comprising D.C. input terminal means adapted to be connected to a source of DC. varying voltage; A.C. input terminal means adapted to be connected to a source of varying A.C. voltage; a set of normally non-conductive transistors comprising a first transistor and a second transistor, the base of said first transistor being connected to said D.C. input terminal means; coupling means connected between said first and second transistors for rendering said second transistor conductive whenever said first transistor is conductive: connecting means for connecting a source of electrical energy to said transistors; output terminal means connected to said second transistor adapted to be connected to an external load; feedback means connected between said base of said first transistor and said output terminal means for providing positive feedback therebetween, said feedback means comprising a voltage divider having a center tap, said voltage divider being connected between the collector of said second transistor and the reference potential; and rectifying means interposed between said A.C. and DC. input terminal means for rectifying an A.C. voltage at said A.C. input terminal means into a DC. voltage at said D.C. input terminal means, said rectifying means comprising a diode connected between said A.C. and DC. input terminal means; resistance means connected between the cathode of the diode and a reference potential of the trigger circuit; and capacitance means connected between the anode of the diode and the reference potential of the trigger circuit, said resistance means providing a current return path for said diode, while said capacitance means charges to a rectified DC. voltage while filtering the A.C. voltage at the anode of said diode to the reference potential, whereby the voltage which appears at said D.C. input terminal means is substantially a DC. voltage devoid of A.C. Voltage, said resistance means comprising a resistor connected to the reference potential through said center of said voltage divider; said transistors being arranged to supply substantially the entire voltage associated with said source of electrical energy to said output terminals when the varying DC. voltage assumes a first predetermined value and said first and second transistors are conductive, while preventing said voltage from being applied to said output terminal means and preventing current from being drawn from the source of electrical energy by the trigger circuit and the external load when the varying DC. voltage assumes a second predetermined value and said transistors are non-conductive. 

1. A trigger circuit comprising D.C. input terminal means adapted to be connected to a source of D.C. varying voltage and having a reference potential associated therewith; a set of normally non-conductive transistors comprising a first transistor and a second transistor, the base of said first transistor being connected to said D.C. input terminal means; coupling means connected between said first and second transistors for rendering said said transistor conductive whenever said first transistor is conductive; connecting means for connecting a source of electrical energy to said transistors; output terminal means connected to said second transistor adapted to be connected to an external load; and feedback means connected between said base of said first transistor and said output terminal means for providing positive feedback therebetween, said feedback means compRising voltage divider means connected between the collector of said second transistor and said reference potential, and resistance means connected between said voltage divider means and said base of said first transistor, said transistors being arranged to supply substantially the entire voltage associated with said source of electrical energy to said output terminals when the varying D.C. voltage assumes a first predetermined value and said first and second transistors are conductive, while preventing said voltage from being applied to said output terminal means and preventing current from being drawn from the source of electrical energy by the trigger circuit and the external load when the varying D.C. voltage assumes a second predetermined value and said transistors are non-conductive.
 2. A trigger circuit as defined in claim 1, wherein said first and second transistors form a complimentary pair of transistors.
 3. A trigger circuit as defined in claim 2, wherein said first transistor is a PNP transistor while said second transistor is a PNP transistor.
 4. A trigger circuit as defined in claim 1, wherein said coupling means comprises a resistor connected between the collector of said first transistor and the base of said second transistor.
 5. A trigger circuit as defined in claim 1, further comprising a single source of electrical energy connected to said connecting means.
 6. A trigger circuit as defined in claim 1, wherein said voltage divider means comprises two series connected resistors forming a center tap, said resistance means comprising a resistor connected between said center tap and said base of said first transistor.
 7. A trigger circuit as defined in claim 1, further comprising A.C. input terminal means adapted to be connected to a source of varying A.C. voltage, and rectifying means interposed between said A.C. and D.C. input terminal means for rectifying an A.C. voltage at said A.C. input terminal means into a D.C. voltage at said D.C. input to terminal means.
 8. A trigger circuit as defined in claim 8, wherein said rectifying means comprises a diode connected between said A.C. and D.C. input terminal means; resistance means connected between the cathode of the diode and a reference potential of the trigger circuit; and capacitance means connected between the anode of the diode and the reference potential of the trigger circuit, said resistance means providing a current return path for said diode, while said capacitance means charges to a rectified D.C. voltage while filtering the A.C. voltage at the anode of said diode to the reference potential, whereby the voltage which appears at said D.C. input terminal means is substantially a D.C. voltage devoid of A.C. voltage.
 9. A trigger circuit as defined in claim 8, wherein said divider means comprises a voltage divider having a center tap, said voltage divider being connected between the collector of said second transistor and the reference potential, said resistance means comprising a resistor connected to the reference potential through said center tap of said voltage divider.
 10. A trigger circuit comprising D.C. input terminal means adapted to be connected to a source of D.C. varying voltage; A.C. input terminal means adapted to be connected to a source of varying A.C. voltage; a set of normally non-conductive transistors comprising a first transistor and a second transistor, the base of said first transistor being connected to said D.C. input terminal means; coupling means connected between said first and second transistors for rendering said second transistor conductive whenever said first transistor is conductive; connecting means for connecting a source of electrical energy to said transistors; output terminal means connected to said second transistor adapted to be connected to an external load; feedback means connected between said base of said first transistor and said output terminal means for providing positive feedbacK therebetween, said feedback means comprising a voltage divider having a center tap, said voltage divider being connected between the collector of said second transistor and the reference potential; and rectifying means interposed between said A.C. and D.C. input terminal means for rectifying an A.C. voltage at said A.C. input terminal means into a D.C. voltage at said D.C. input terminal means, said rectifying means comprising a diode connected between said A.C. and D.C. input terminal means; resistance means connected between the cathode of the diode and a reference potential of the trigger circuit; and capacitance means connected between the anode of the diode and the reference potential of the trigger circuit, said resistance means providing a current return path for said diode, while said capacitance means charges to a rectified D.C. voltage while filtering the A.C. voltage at the anode of said diode to the reference potential, whereby the voltage which appears at said D.C. input terminal means is substantially a D.C. voltage devoid of A.C. Voltage, said resistance means comprising a resistor connected to the reference potential through said center of said voltage divider; said transistors being arranged to supply substantially the entire voltage associated with said source of electrical energy to said output terminals when the varying D.C. voltage assumes a first predetermined value and said first and second transistors are conductive, while preventing said voltage from being applied to said output terminal means and preventing current from being drawn from the source of electrical energy by the trigger circuit and the external load when the varying D.C. voltage assumes a second predetermined value and said transistors are non-conductive. 